Yes they can - grey dice are dice, you just don't need to actually roll them. The point of shell shock is as follows: for synergy reasons you sometimes want you slowpokes turtle shelled before they activate or when they cannot activate anymore this turn. Examples: you run somewhere with a couple of slowpokes running forfeits all their actions they cannot become turtle shells by themselves. Next you use shell shock to turtles he'll all of them. And finally you use your crusher's turtle tremor taking advantage of all the shells in the area. Or second example. You shell shock two slowpokes that have not activated yet. Your Roller twacks! Them somewhere, they activate and still can move their full distance and attack. And there are more potential uses...